Athens: Greece`s Prime Minister Alexis Tsipras will plead with European counterparts at a summit in Brussels Thursday to release vital funds to help his debt-laden country stave off a looming cash crunch.

European Union leaders will also debate whether to extend economic sanctions against Russia over the conflict in Ukraine beyond July, although it is likely they will delay the decision until later in the year.

The twin crises are set to dominate the two-day summit, with Brussels still scrambling for answers to some of the most severe economic and strategic tests in the history of the 28-nation bloc.

While Greece is not officially on the agenda, an EU official said it was the "elephant in the room".

EU President Donald Tusk has convened special talks on the sidelines of the summit on Thursday night among Tsipras, German Chancellor Angela Merkel and French President Francois Hollande, at the request of the Greek premier, Tusk`s spokesman told AFP.

The meeting will also be attended by European Commission head Jean-Claude Juncker, European Central Bank chief Mario Draghi and Jeroen Dijsselbloem, the Dutch finance minister who chairs the euro single currency group.

The talks will give embattled leftist leader Tsipras a chance to convince Merkel and Hollande, the key political duo in the eurozone, to accept Greece`s promises of reform in return for relaxing the terms of its bailout programme.Fears are growing of a potentially disastrous Greek exit from the euro -- whether by accident or design -- with Athens facing the possibility of being unable to pay its bills by Friday.

Amid increasingly bitter rhetoric on both sides, the Greek parliament on Wednesday adopted a "humanitarian crisis" bill aimed at helping the poor, ignoring apparent pressure from the EU to halt the legislation.

Germany`s Finance Minister Wolfgang Schaeuble, one of the sternest critics of the new Greek government`s bid to abandon austerity, meanwhile warned Wednesday that time was "tight" for Athens.

Greece wants the last tranche of its 240-billion-euro ($255-billion) EU-IMF bailout, amounting to about seven billion euros, but Brussels is refusing until it first approves Athens`s new reform package.

Friday brings a key debt deadline when Greece must pay 300 million euros to the IMF and redeem 1.6 billion euros in treasury bills, with a European diplomat saying there were questions about "even how to get through the next few days."

Tsipras has caused jitters in Europe by announcing a surprise visit to Moscow next month, despite relations between the EU and Russia plunging to their worst state since the Cold War thanks to the crisis in Ukraine.The state of EU economic sanctions imposed on Russia last July after the shooting down of Malaysia Airlines flight MH17 over rebel-held eastern Ukraine is one of the key issues on the summit agenda for Thursday.

A rollover of the sanctions until the end of 2015 is the "most likely option", but it is "unlikely that will be agreed" this time, diplomats said, with the leaders likely to postpone any decision until their next summit in June.

An extension until December 31 would bring the sanctions in line with the timetable of last month`s Minsk peace deal, which led to a shaky ceasefire between government forces and pro-Kremlin rebels in eastern Ukraine.

New sanctions "won`t happen", with the EU and US both believing they would only be triggered by a major ceasefire breach such as a rebel assault on the strategic port city of Mariupol, diplomats said.

In their closing statement on Friday, the leaders are expected to issue "strong language" that will "point the way to a formal decision" on a rollover at a later date, sources said.

They are also expected to back the creation of a special media team aimed at countering Russian "disinformation" over Ukraine, a recognition of the fact that the West has largely been outmanoeuvred in the propaganda war.

A so-called "energy union" plan for the bloc will also be discussed Thursday, especially measures to reduce its dependence on Russian gas.

Libya will be discussed on Friday, although talk of a possible EU security mission to support any possible unity government is "premature", officials said.